1. Field of the Invention
The present invention relates to a horizontal axis wind turbine and a method for controlling a horizontal axis wind turbine.
2. Description of Related Art
In recent years, horizontal axis wind turbines have been proposed and in practical use in order to generate electric power from natural wind. Such a horizontal axis wind turbine is provided with a yaw sensor, such as a yaw vane, for measuring the direction of the wind blowing against the rotor (wind direction). A control device of the horizontal axis wind turbine performs “yaw control” of starting rotation of the rotor as well as rotates the rotor axis in a substantially horizontal plane in such a way that the rotor faces the wind direction measured by the yaw sensor, i.e., the yaw angle (the angle between the direction of the rotor axis and the wind direction) is made to converge to about 0 degree.
By the way, most of commercial wind turbines used nowadays is an upwind turbine in which a rotor 100 is located on the windward side as shown in FIGS. 6-8. A yaw sensor 200 of the upwind turbine is generally located from a substantial central portion to a posterior portion of the rotor axis direction of the upper portion of a nacelle 300.
However, wind turbines are often installed in complex topography and, in the topography, an upflow wind (blowing up) is often generated. Therefore, when the yaw sensor 200 is located on the upper portion of the nacelle 300 as shown in FIGS. 6-8, measurement accuracies of the wind direction and the yaw angle is degraded significantly by an effect of the nacelle 300. As a result of this, there is a problem that an error occurs between the yaw angle φ measured by the yaw sensor 200 and the actual yaw angle φ and so the yaw control of the rotor 100 is not performed accurately.
FIG. 9 shows measurement errors of a yaw angle caused by such an upflow angle and is a graph showing results of a wind tunnel test of measurement values of the yaw angle in case of locating the yaw sensor 200 on the upper portion of the nacelle 300 as shown in FIGS. 6-8. In FIG. 9, the horizontal coordinate is a value of a yaw angle φ in case of an upflow angle of 0 degree (hereinafter referred to as a “true yaw angle value”) and the vertical coordinate is a difference between a value of the yaw angle φ measured by the yaw sensor 200 in case of an upflow angle of 0 degree to +30 degree and a value of the yaw angle φ in case of an upflow angle of 0 degree (hereinafter referred as to a “yaw angle measurement error”.
For example, in the case that an upflow having an upflow angle of +30 degree is blowing against the rotor 100, a yaw angle measurement error (the vertical coordinate) changes from about −30 degree to about +10 degree by about +40 degree corresponding to a true yaw angle value (the horizontal coordinate) changing from 0 degree to +10 degree by +10 degree (see FIG. 9: curve E). That is, a value of the yaw angle φ measured by the yaw sensor 200 is measured as changing by about +50 degree, corresponding to a value of the actual yaw angle φ changing by +10 degree. Therefore yaw control of the rotor 100 is repeated although the change of the wind direction is slight.
Such measurement errors of a yaw angle caused by an upflow angle can be reduced slightly by locating the yaw sensor 200 at a high position not to be affected by the nacelle 300. However, there is a problem that the yaw sensor 200 shakes easily, its durability is degraded and a more cost is required because this means makes a supporting rod 210 supporting the yaw sensor 200 very long.